The best way to handle the situation is to start a new Texas LLC, get a new EIN, and don't worry about your suppliers and clients. They will understand. If any of them deal with companies on a regular basis, they won't mind at all. Is there a way to do it differently? Absolutely, but you asked for the best way. Here's why I say it's best this way:
1. EIN is issued by the IRS. Your EIN is linked to a company name, which (I presume) passes it's tax information along to the company owners. If you start a new company, you'll need a new EIN, period. If you have a complete change of ownership (which I know you're not planning on right now, but I will explain in a moment), then you'll likewise need a new EIN. If you want to keep the same EIN, you'll need to keep the same company and at least one of the same owners.
2. Keeping the same company means you'll need to either pay someone in Pennsylvania (to be your registered agent) or maintain an office there yourself. These are both pricey options if you ask me, so I don't think this is a good option. If you do decide to do this, there's another problem: you will have to pay both the State of PA and the State of TX whether you decide to start a second company or register the first to do business in TX. In my experience in TX, you're better off having a TX company than a northeastern company, but that's my own opinion based on business experience, not legal-related. Still, if you do decide to keep the PA company, and since you'd be paying in both states anyway, I'd suggest opening a TX LLC to be owned by the PA LLC. That way you could keep people paying the PA LLC with the same EIN. That is not a tax-beneficial setup, but again I don't recommend it anyway. If you were to do it the other way around (TX company owns PA company) then you'd need a new EIN anyway for the PA company, because it would have a new owner).
Again, as I said, the best option is just to let the PA LLC go, open a new Texas LLC, and get a new EIN for it.
Good luck and please feed positive feedback if you feel I've earned it.